The present invention relates to a communications system using quantum cryptography.
As described in the present applicant's earlier applications Nos. PCT/GB93/02637 and PCT/GB93/02075, quantum cryptography is a technique for the distribution of a key for use in the subsequent encryption or decryption of transmissions in a communications system. The key is distributed by modulation of a single-photon signal. Any eavesdropper intercepting the single-photon signal disturbs the statistics of the received data, and so can be detected. The term "single-photon signal" encompasses any signal having the required quantum properties. It may be a true single-photon generated, for example, by parametric down-conversion. Such a technique is described and claimed in our earlier above-cited applications. Such true single-photon sources have been found however to be difficult to construct and run reliably. In general therefore it is preferable to take a "classical" source, such as a laser diode (LED) and to attenuate severely the output from such a source to a level such that in a given time slot there is in general no more than one, and on average very much less than one photon.